Generally, a multifocal lens is required to provide a certain power for far vision and different, usually greater (more positive), powers for mid and near vision, the additional power for mid and near vision sometimes being referred to as “mid-add” and “near-add”, which is usually expressed in dioptres. Multifocal lenses with two foci are referred to as bifocal.
Compared with monofocal ophthalmic lenses, multifocal ophthalmic lenses offer the advantage of reduced spectacle dependency, whereas patients with monofocal lenses generally need reading spectacles. In an ideal situation, the patient will have good vision in distance and near, while the depth of focus will enable vision in the intermediate. In this situation, the patient doesn't need spectacles in any situation. However, since a multifocal lens splits the available light into two or more foci, the visual quality in each focus is somewhat reduced. When a distant object is focused on the retina, a blurred image is superimposed due to the presence of the additional foci and vice versa, which obviously reduces the image quality. The reduced visual quality can be divided in reduced contrast sensitivity and appearance of optical phenomena, like straylight and halos. Moreover a patient has to undergo a learning period after implantation, as the two (or more) simultaneous images displayed on the retina can be confusing in the beginning. In most cases, the blurred image is discarded by the human visual perception and retinal processing system.
Usually, multifocal lenses are designed according to one or more of the following optical principles:                1. Diffractive type: conventional refractive lens combined with diffractive optics that splits light into two or more focal points.        2. Refractive optics with annular zones/rings with different radii of curvatures.        
Examples of bifocal and multifocal intraocular lenses are disclosed in U.S. Pat. No. 4,642,112 and U.S. Pat. No. 5,089,024. Examples of commercially available multifocal lenses are: model CeeOn® model 811 E, Pharmacia, Kalamazoo, Mich. and SA 40, AMO, Irvine, Calif. The former is based on diffractive optics, whereby light is partitioned into two focal points, one for distance vision and one for near vision. The latter is a distance-dominant, zonal-progressive, multifocal optic with a 3.5-diopter near-add.
After IOL implantation, any remaining defocus (sphere) and astigmatism (cylinder) can be corrected by spectacles or contact lenses. Beside first order defocus and astigmatism of the eye a number of other vision defects could be present. For example aberrations of different orders occur when a wavefront passes a refracting surface. The wavefront itself becomes aspheric when it passes an optical surface that has imperfections, and vision defects occur when an aspheric wavefront falls on the retina. Both the cornea and the lens in the capsular bag contribute thus to these types of vision defects if they deviate from being perfect or perfectly compensating optical elements. The term aspheric will in this text include both asphericity and asymmetry. An aspheric surface could be either a rotationally symmetric or a rotationally asymmetric surface and/or an irregular surface, i.e. all surfaces not being spherical.
Recently, in studies on older subjects, it has been discovered that the visual quality of eyes having an implanted monofocal IOL, having spherical lens surfaces (hereafter referred to as a conventional intraocular lens (CIOL)) is comparable with normal eyes in a population of the same age. Consequently, a 70 year old cataract: patient can only expect to obtain the visual quality of a non-cataracteous person of the same age after surgical implantation of an intraocular lens, although such lenses objectively have been regarded as optically superior to the natural crystalline lens. This result is explained by the fact that CIOLs are not adapted to, compensate for defects of the optical system of the human eye, namely optical aberrations.
In order to improve the performance of implanted intraocular lenses, efforts have been made to provide intraocular lenses for implantation that at least partly compensates for such aberrations (Reduced Aberration TOL, or RAIOL). The applicant's own application WO 01/89424 discloses an ophthalmic lens providing the eye with reduced aberrations, and a method of obtaining such. The method comprises the steps of characterizing at least one corneal surface as a mathematical model, calculating the resulting aberrations of said corneal surface(s) by employing said mathematical model, selecting the optical power of the intraocular lens. From this information, an ophthalmic lens is modeled so a wavefront arriving from an optical system comprising said lens and corneal model obtains reduced aberrations in the eye. The ophthalmic lenses as obtained by the methods are thus capable of reducing aberrations of the eye.
Of current multifocal lenses, the optical quality is lower than for current monofocal lenses. This shows in contrast sensitivity measurements on pseudophakic patients. As the visual quality of multifocal lenses is relatively low, even minor improvements in optical quality will lead to visible improvements.
Both WO 00/76426 and U.S. Pat. No. 6,457,826 mentions the possibility to make an aspheric BIOL. WO 00/76426 does not disclose use of any specific aspheric characteristic in the lens, but just mentions the possibility to combine an asphere with a diffractive pattern. However, U.S. Pat. No. 6,457,826 states that optical corrections can be made by aspherizing an IOL surface, but it is not at all described how this could be done.
In view of the foregoing, it is therefore apparent that there is a need for multifocal ophthalmic lenses that are better adapted to compensate the aberrations caused by the individual surfaces of the eye, such as the conical surfaces, and capable of better correcting aberrations other than defocus and astigmatism, as is provided with conventional multifocal intraocular lenses.